The invention pertains to a power amplifier circuit, and in particular to tube power amplifier with a distortion detection circuit and a responsive control circuit for reducing distortion.
Power amplifier circuits with distortion reduction means are known, as exemplified in U.S. Pat. No. 4,318,053. The '053 patent, known as Distortion Detection Technique (DDT), discloses a solid state amplifier with automatic distortion control. The amplifier is provided with a detector between the amplifier output and feedback ports for detecting a nonlinear condition with a balanced differential circuit connected to the detector and feeding a full wave threshold detector which charges a storage capacitor proportionally to the percent clipping of the output signal of the amplifier. The storage capacitor is discharged into an input variable gain amplifier in the amplifier input circuit to reduce the gain for minimizing clipping.
The DDT circuit in the '053 patent is for a solid state amplifier circuit. It is desirable to employ DDT for tube type power amplifiers to thereby avoid clipping when a signal is fed into the input of the power amplifier that would normally be sufficient to clip the power amplifier.